The retinal pigment epithelium (RPE) is a single layer of specialized neuro-epithelial cells juxtaposed between the choroid and the retina of the eye. Several hormones and neurotransmitters existing either in the choroid (11-14) or in the retina (15, 16) are found to elevate the intracellular cyclic AMP concentration of the RPE from the chick embryos. The present study propose to examine the possibility that the hormones and neurotransmitters of the choroidal and/or retinal origin may function as modulators of the RPE functions. Among the modulators of the cyclic AMP pools of the RPE, the vasocative intestinal peptide (VIP) is the most effective one. The VIP receptor on the RPE is therefore to be used as a model in identifying the roles played by the hormones and neurotransmitters in modulating the functions of the RPE. The proposed study will characterize the VIP receptor in the following aspects relevant to establishing a physiological role for this receptor in the RPE cells: 1) Localization of the VIP receptors on the apical and/or the basolateral membrane, 2) Functional studie of the receptors: the effect of VIP on transport of electrolyte and macromolecule, 3) The mechanism of VIP action of the RPE cells: beyond stimulation of the adenylate cyclase and phosphorylation of the cellular proteins, 4) Visualization of the VIP receptors on the RPE cells intact in the eye by autoradiographic methods. The present proposed study represents an effort aiming toward understanding the dynamic relatioships that existing among the choroid the RPE and the retina, and the conditions that lead to breakdown of these relationships, such as the retinal detachment.